1. Field of the Invention
The present invention relates to a voltage detecting device for battery modules to detect the voltage of each battery module of a battery pack.
2. Description of the Related Art
For example, an electric power for an electric vehicle and a hybrid vehicle is supplied from a battery pack comprised of a number of cells connected in series. Accordingly, the voltage of a battery module which is a series circuit of a plurality of cells is normally monitored.
According to a method of detecting the voltage of the battery module of the battery pack, there is known a measuring method by which an output voltage of the battery module to be measured is applied to one capacitor by sequentially switching switches, so that the capacitor is charged, and a voltage across the capacitor is measured by a differential amplifier.
When an A/D (analog to digital) converter converts an analog output signal of the differential amplifier into a digital signal, aliasing occurs due to noise caused by frequencies higher than half a sampling frequency. Preferably, an anti-aliasing filter is applied between the switch and a battery module to be measured so as to prevent the aliasing.
JP 2005-003618A discloses a voltage detecting circuit which includes an anti-aliasing filter for each battery module. The anti-aliasing filter is a low-pass filter composed of a resistor and a capacitor.
However, the technology disclosed in JP 2005-003618A has the problem in occurring the difference in frequency response between the battery modules and having an inhomogeneous filter characteristic with respect to each battery module since a resistor connected between the battery modules is commonly used. In other words, when two sets of resistors are provided for each battery module, the frequency response is equalized. However, when the resistor between the battery modules is commonly provided in order to reduce the number of electronic parts, the frequency response becomes inhomogeneous. In this case, even if there is no difference in an output voltage waveform of each battery module, there appears the difference in the voltage waveform of each battery module through a filter. Accordingly, it is erroneously determined that the battery module is in an irregular condition. In particular, since the voltage detecting circuit of JP 2005-003618A amends the difference in frequency response between the battery modules on the basis of a constant value of electronic parts, the constant value needs to be strictly determined. If the constant value is erroneously determined, an error resulting from variance of the constant value is considerably large. For example, if a photo MOS relay having a predetermined delay time in switching is provided for a switch, a sampling frequency of switching cannot be raised due to a long delay time. Consequently, the voltage detecting device including the photo MOS relay is affected by the noise of a relatively low frequency.